Fluid Mechanics and Thermodynamics of Turbomachinery, 5e

Two-dimensional Cascades 85
FIG. 3.25. Profile loss coefficients of turbine nozzle and impulse blades at zero
incidence (t/l = 20%; Re = 2 ¥ 10 5 ; M < 0.6) (adapted from Ainley and Mathieson 1951).
where all the values of Yp are taken at the same space–chord ratio and flow outlet
angle. If rotor blades are being considered, put 3b2 for a1 and b3 for a2. Equation
(3.46) includes a correction for the effect of thickness–chord ratio and is valid in
the range 0.15 t/l 0.25. If the actual blade has a t/l greater or less than the
limits quoted, Ainley recommends that the loss should be taken as equal to a blade
having t/l either 0.25 or 0.15. By substituting a1 = a2 and t/l = 0.2 in eqn. (3.46),
the zero incidence loss coefficient for the impulse blades Yp(a1=a2) given in Figure
3.25 is recovered. Similarly, with a 1 = 0 at t/l = 0.2 in eqn. (3.46) gives Yp(a1= 0) of Figure
3.25.
A feature of the losses given in Figure 3.25 is that, compared with the impulse
blades, the nozzle blades have a much lower loss coefficient. This trend confirms
the results shown in Figure 3.14, that flow in which the mean pressure is falling